This grant's original goals were to determine the suitability of committed cells such as cardiomyocytes, skeletal muscle cells, smooth muscle cells and fibroblasts for cardiac repair. Our original specific aims were: 1) To determine the basis for electromechanical coupling of cardiac and skeletal muscle cells in culture, and determine whether such coupling could be induced in skeletal muscle grafts in the heart. 2) To test the hypothesis that skeletal muscle grafts in the heart are ischemia-resistant and contain muscle stem cells (satellite cells) capable of generating new muscle after myocardial infarction. 3) To determine whether cardiomyocyte or fibroblast grafting prevents ventricular dilation and interstitial fibrosis after infarction. Although significant progress has been made on these aims, since this grant's inception there has been an explosion of new information about the potential of adult and embryonic stem cells to effect myocardial repair. Studies supported by this grant have shown, for example, that adult humans have the ability to repopulate multiple cell lineages within their hearts, although cardiomyocyte repopulation is too low to be physiologically significant. Most recently, we have begun to explore using human embryonic stem cells (hESCs) for myocardial repair. We have confirmed reports in the literature that indicate hESC-derived cardiomyocytes have surprisingly high proliferation rates in vitro, much higher than has been seen with model species such as rates and mice. The studies proposed in this supplemental application have two specific aims: Aim 1) Determine the basal levels of hESC-derived cardiomyocyte proliferation in vitro. Aim 2) Characterize the mitogenic effects of various molecules in hES-derived cardiomyocytes in vitro to identify key pathways involved in hESC-derived cardiomyocyte proliferation. These studies will establish conditions for studying cell cycle activity in human cardiomyocytes, and may provide insights into the pathways controlling proliferation. If such pathways can be understood, it could provide a rational basis to control cardiomyocyte proliferation for cardiac repair applications. [unreadable] [unreadable]